1. Field of the Invention
The present invention relates to a distance measuring system for measuring the distance between two objects using an electromagnetic wave.
2. Description of the Related Art
Heretofore, an FM-CW radar system, for example, has been known as a distance measuring system for measuring the distance between two objects using an electromagnetic wave. The FM-CW radar system transmits a frequency-modulated electromagnetic wave from one object (hereinafter referred to as a "measuring object") toward another object (hereinafter referred to as a "measured object"). A reflected electromagnetic wave from the measured object is received by the measuring object. A signal representing the reflected electromagnetic wave which is received by the measuring object is delayed from a signal representing the electromagnetic wave which is emitted from the measuring object, by a time lag that corresponds to a period of time required for the electromagnetic wave to travel back and forth between the objects. Comparison of these transmitted and received signals on the same time base shows that there is developed a frequency difference between the transmitted and received signals which is proportional to the distance between the objects. The FM-CW radar system detects such a frequency difference and determines the distance between the objects based on the detected frequency difference.
In order to determine the distance between the objects, it is therefore necessary for the FM-CW radar system to detect a frequency difference between the received signal which represents the electromagnetic wave reflected directly from the measured object and the transmitted signal. Actually, however, the received signal contains various components which represent indirect electromagnetic waves which return from the measured object via other objects and are received by the measuring object. Therefore, the FM-CW radar system is required to extract only a component of the received signal which represents the electromagnetic wave reflected directly from the measured object, in order to accurately measure the distance between the objects. To meet such a requirement, the FM-CW radar system has to effect complex signal processing such as a frequency analysis, etc., and is necessarily complex in structure and expensive to manufacture. When the FM-CW radar system effects a frequency analysis on the received signal and extracts a spectrum which corresponds to the directly reflected electromagnetic wave and is required to determine the distance between the objects, the spectrum contains various frequency components produced due to the configuration of the reflecting surface of the measured object and the positional relationship thereof to the measuring object. The various frequency components contained in the spectrum make it difficult to specify the frequency that corresponds to the distance between the objects, and hence to accurately determine the distance between the objects.